escape the % operator in string formatting

CHANGELOG

@@ -1,6 +1,7 @@
 Changese between 2.2.3 and ...
 -----------------------------------
 
+- fix error message for rewritten assertions involving the % operator
 - fix issue 126: correctly match all invalid xml characters for junitxml
   binary escape
 - fix issue with unittest: now @unittest.expectedFailure markers should

_pytest/assertion/rewrite.py

@@ -297,7 +297,7 @@ binop_map = {
     ast.Mult : "*",
     ast.Div : "/",
     ast.FloorDiv : "//",
-    ast.Mod : "%",
+    ast.Mod : "%%", # escaped for string formatting
     ast.Eq : "==",
     ast.NotEq : "!=",
     ast.Lt : "<",

testing/test_assertrewrite.py

@@ -195,6 +195,10 @@ class TestAssertionRewrite:
             y = -1
             assert x + y
         assert getmsg(f) == "assert (1 + -1)"
+        def f():
+            x = range(10)
+            assert not 5 % 4
+        assert getmsg(f) == "assert not (5 % 4)"
 
     def test_call(self):
         def g(a=42, *args, **kwargs):